That's a really interesting way to think about that, the notion that online behavior cannot be disruptive. But I would respectfully disagree. Yes, online posts or emails can be ignored but not until after one has read/viewed inappropriate content to decide whether to ignore it. How is that supposed to work?
I once had a student post blatantly obscene & only marginally relevant comments in a discussion forum (I will not repeat it here, but it was, let's just say, a very colorful description of certain sexual practices). I deleted his post and sent him a note. He turned out to be apologetic and was fine for the rest of the semester and continued with a high level of appropriate participation.
But if he had insisted on continuing such posts, I would not have hesitated to block him from the class and then send the case along to student affairs. It's not a freedom of speech issue. Just like in a live classroom, some forms of expression are appropriate, others are not. Indeed, in the online environment, it's easier to let student discussion wander off track because there is no class "time" to waste, but there still have to be basic limits in place. It's not the job of other students to memorize which student is the one who sometimes posts offensive things and then be sure never open a post from that student. And as the instructor, I have no right to ignore offensive content--I have to moderate every discussion and grade every submission.
At my institution, the same rules for dropping out a student or barring access apply to the classroom and online environments. If an instructor entirely drops a student from an online class, that student can no longer log in at all. But instructors also have the option to temporarily block student access from the course content--they can still get in and check grades, email the instructor, etc., but not actively participate.
Dropping or blocking an online student is most often done for lack of attendance and rarely in response to disruptive behavior. I believe we recently had a case where a student was blocked because he was using the in-class email to repeatedly send inappropriate messages to other students. I don't know the details of the case.
Well, we can warn students that their comments are inappropriate, but our legal counsel takes a dim view of deleting comments or blocking students. I've been teaching at least one online course per term now since 2006 and I haven't seen anything all that shocking.
About three years ago, I did have two students did get into it a little. Very tame stuff by greater internet standards. One student complained that hu felt insulted by the other student. I posted our netiquette guidelines, told both of them to play nicely, and made it clear to the insulted student that sometimes feelings get hurt and that's just a part of life. They both toned it down.